Relative volatility

Relative volatility

Relative volatility

Relative volatility is a measure comparing the vapor pressures of the components in a liquid mixture of chemicals. This quantity is widely used in designing large industrial distillation processes. In effect, it indicates the ease or difficulty of using distillation to separate the more volatile components from the less volatile components in a mixture. By convention, relative volatility is usually denoted as α.
Relative volatilities are used in the design of all types of distillation processes as well as other separation or absorption processes that involve the contacting of vapor and liquid phases in a series of equilibrium stages.
Relative volatilities are not used in separation or absorption processes that involve components reacting with each other (for example, the absorption of gaseous carbon dioxide in aqueous solutions of sodium hydroxide).
  
Definition

For a liquid mixture of two components (called a binary mixture) at a given temperature and pressure, the relative volatility is defined as

where:
α	= the relative volatility of the more volatile component i to the less volatile component j
yi	= the vapor-liquid equilibrium concentration of component i in the vapor phase
xi	= the vapor-liquid equilibrium concentration of component i in the liquid phase
yj	= the vapor-liquid equilibrium concentration of component j in the vapor phase
xj	= the vapor-liquid equilibrium concentration of component j in the liquid phase
(y / x)	= K   commonly called the K value or vapor-liquid distribution ratio of a component

When their liquid concentrations are equal, more volatile components have higher vapor pressures than less volatile components. Thus, a K value (= y / x) for a more volatile component is larger than a K value for a less volatile component. That means that α ≥ 1 since the larger K value of the more volatile component is in the numerator and the smaller K of the less volatile component is in the denominator.
α is a unit less quantity. When the volatilities of both key components are equal, α = 1 and separation of the two by distillation would be impossible under the given conditions because the compositions of the liquid and the vapor phase are the same (azeotrope). As the value of α increases above 1, separation by distillation becomes progressively easier.

Thus, for the distillation of any multi-component mixture, the relative volatility is often defined as

 

 

Air-Sensitive Vacuum Distillation

Air-Sensitive Vacuum Distillation


Some compounds have high boiling points as well as being air sensitive. A simple vacuum distillation system as exemplified above can be used, whereby the vacuum is replaced with an inert gas after the distillation is complete. However, this is a less satisfactory system if one desires to collect fractions under a reduced pressure. To do this a "pig" adaptor can be added to the end of the condenser, or for better results or for very air sensitive compounds a Perkin triangle apparatus can be used.

The Perkin triangle, has means via a series of glass or Teflon taps to allows fractions to be isolated from the rest of the still, without the main body of the distillation being removed from either the vacuum or heat source, and thus can remain in a state of reflux. To do this, the sample is first isolated from the vacuum by means of the taps, the vacuum over the sample is then replaced with an inert gas (such as nitrogen or argon) and can then be stoppered and removed. A fresh collection vessel can then be added to the system, evacuated and linked back into the distillation system via the taps to collect a second fraction, and so on, until all fractions have been collected.

Vacuum Distillation

Vacuum Distillation

Some compounds have very high boiling points. To boil such compounds, it is often better to lower the pressure at which such compounds are boiled instead of increasing the temperature. Once the pressure is lowered to the vapor pressure of the compound (at the given temperature), boiling and the rest of the distillation process can commence. This technique is referred to as vacuum distillation and it is commonly found in the laboratory in the form of the rotary evaporator.

This technique is also very useful for compounds which boil beyond their decomposition temperature at atmospheric pressure and which would therefore be decomposed by any attempt to boil them under atmospheric pressure.